The present invention relates to a fixing device for a printer, facsimile apparatus, copier or similar electrophotographic image forming apparatus and more particularly to an induction heating type fixing device using electromagnetic induction. Further, the present invention relates to an induction coil for use in the induction heating type fixing device.
Generally, an image forming apparatus includes a fixing device for fixing a toner image on a paper sheet or similar recording medium. One of conventional fixing device uses a heat roller to be heated and a press roller pressed against the heat roller. The heat roller and press roller fix a toner image on a paper sheet with heat and pressure while conveying the paper sheet. A halogen lamp, halogen heater or similar heat source has traditionally been disposed in the heat roller for heating the heat source to a preselected temperature necessary for fixation.
It is a common practice with the above-described fixing device to heat the heat roller to a preselected surface temperature, e.g., 180° C. and then maintain it at a temperature that is about 50% to 90% of the above temperature, e.g., 120° C. This allows a person to use the image forming apparatus without wasting time. However, even when the fixing device is held in a stand-by state at, e.g., 120° C., it naturally consumes much power. From the energy saving standpoint, the fixing device should consume a minimum of energy even in the stand-by state.
In light of the above, there have been proposed various fixing systems featuring a short warm-up time and therefore allowing power supply to be shut off when in a stand-by state. Particularly an induction heating type fixing system is attracting increasing attention because it heats a heat roller, which is formed of conductive metal, by using eddy current derived from an electromagnetic wave.
In an induction heating type fixing device, if the range over which an induction coil is wound differs from a sheet passing width, then so-called hot offset occurs due to excessive temperature elevation in ranges that a paper sheet does not pass. Japanese Patent Laid-Open Publication No. 2000-133627, for example, discloses an induction heating type fixing device capable of obviating hot offset and reducing wasteful power consumption. The fixing device disclosed includes a main coil and an auxiliary coil assigned to a range covering small paper sizes and a range outside of the above range, respectively. The main coil and auxiliary coil are wound round a single bobbin and controlled independently of each other as to current supply. The gap between the coils and a heat roller should preferably be as small as possible in order to efficiently heat the heat roller. To reduce the gap, the end portions of the auxiliary coil are laid inside of the bobbin.
In the configuration taught in the above-mentioned document, the auxiliary coil is positioned at opposite end portions of the bobbin while the main coil is positioned at the center of the same. This brings about a problem that the main coil is laid inside of the bobbin, making it difficult to deal with the end portions of the coils. Further, when any part of the coils snaps, all the coils must be rewound. A period of time necessary for rewinding work is so long, the coils are bodily replaced, i.e., even the coil not snapped is discarded.
On the other hand, the warm-up time of the fixing device can be reduced if the thermal capacity of the heat roller is reduced. For this purpose, the wall thickness of the heat roller may be reduced, as proposed in the past. This, however, brings about another problem that substantially no heat conduction occurs in the heat roller in the axial direction because the heat roller has a core whose thermal conductivity is relatively low. To solve this problem, there has been proposed a system in which a plurality of coils are selectively energized in accordance with sheet size. This allows only necessary part of the heat roller to be heated and thereby further saves energy.
The plurality of coils, however, require a number of leads to be laid between the heat roller and the coils. Therefore, to protect the coils from damage ascribable to, e.g., short-circuiting, a sufficient gap is necessary between the heat roller and the coils and leads. Such a gap increases the distance between the coils and the heat roller and thereby aggravates thermal conversion efficiency. Moreover, a number of leads increase the number of wiring steps and therefore production cost.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-10901, 10-20704, 10-153918, 10-282826, 11-316509, 2000-105518, and 2000-56598.